1. Field of the Invention
The present invention relates in general to a winding machine for winding an electric wire around a winding bobbin to form a perfect layer coil. In particular, the invention concerns a tension control apparatus for controlling tension applied to the electric wire being fed to the winding bobbin so that the tension remains substantially constant independently of variation of the tension produced in one rotation of the winding bobbin or variation in the rotating speed of the winding bobbin.
2. Description of the Prior Art
For forming a perfect layer coil by winding a continuous electric wire around a winding bobbin of a circular cross-section, it becomes necessary to carry or step up the wire being wound to a next outer layer position at both ends of the winding bobbin successively thereby to coordinate the superposed positions of the electric wire in the individual winding layers. To this end, the rotating speed of the bobbin is decelerated when the winding position of the wire reaches one or several turns short of either one end of the bobbin, and the winding position is stepped up to the next upper layer position. Then, after winding of the wire for a single or several turns, the rotation speed of the bobbin is again accelerated to a predetermined constant speed. As the consequence of the deceleration of the rotating speed of the bobbin effected when the winding position of the wire is stepped up to the next outer winding layer, there is produced a reduction in tension applied to the wire and eventually a sag, because the wire fed out from a supply reel at a high speed before the deceleration will tend to remain at the same speed due to inertia of the wire portion spanned between the bobbin and the supply reel as well as inertia of pulleys for guiding the electric wire, i.e. the wire is fed out from the supply reel in a larger quantity than the wire is wound around the bobbin. On the other hand, upon acceleration of the winding bobbin, tension on the spanned wire will be abruptly increased due to a drawing force exerted to the wire for accelerating the feeding speed thereof.
In the case of the winding machine in which a bobbin having a winding portion of a rectangular cross-section is employed, tension applied to the electric wire undergoes rapid variation every time the winding position of the electric wire moves over a corner of the rectangular bobbin during every revolution of the bobbin in addition to variations occurring at both ends of the bobbin.
In the winding of a perfect layer coil, the rapid variations in the tension of the wire will involve deterioration in the wire alignment among the turns of coil wound on a bobbin. More specifically, an abrupt or rapid reduction in the tension applied to the wire will result in generation of slack or a sag, making it difficult to wind the electric wire correctly on or along the wire already wound around the bobbin. On the other hand, an abrupt or rapid increase in the tension of the wire will result in the wire being undesirably stretched to give rise to variation in the resistance value of the wire for a unit length, delamination of a coated insulation layer and possibly breakage of the wire in the worst case.
As will be appreciated from the above discussion, variation in the tension of the electric wire being fed during the winding operation to form a coil will bring about various problems, it is desirable that the tension of the wire should be maintained as constant as possible regardless of variation of the tension produced in one rotation of the bobbin or variation rotating speed of the winding bobbin.
As an attempt to overcome the difficulties described above, there has been heretofore proposed a tension control apparatus which is composed of a brake reel around which a continuous electric wire as drawn from a supply reel is wound for a desired number of turns to thereby apply a tension to the wire, brake means for applying to the brake reel a braking force corresponding to an input brake voltage, tension adjusting means including a roller rotatably supported on a pivotally mounted arm so as to be positioned between the brake reel and the winding bobbin, which roller serves to engage the wire in such manner that the arm is swung by following variation in the tension of the wire being fed to the winding bobbin to thereby absorb variations in the tension of wire, and a differential amplifier circuit for producing the brake voltage in dependence on the swinging magnitude of the arm, whereby the tension of the wire is controlled as a function of the swinging movement of the arm.
The prior art tension control apparatus mentioned above is disadvantageous in that the controlling operation is effected only with a delay, rendering it impossible to maintain the tension constant, since there is involved a large amount of time delay between occurrence of variation in the tension of the wire and the application of the braking force to the brake reel due to the fact the arm is first swung in response to variation in the tension of the wire to actuate the differential amplifier for deriving a requisite brake voltage which is then applied to the brake means to be subsequently converted into a corresponding braking force to be applied to the brake reel. Particularly when a coil of a rectangular cross-section is formed at a high speed, the tension controlling apparatus can not follow rapid variations in the tension of the wire with reasonable reliability. Besides, since the swinging amplitude of the sensing arm becomes increased as variation in the tension becomes more remarkable, a relatively long time is required for the tension of the wire to become stabilized because of generation of attenuating oscillation of the arm. Further, when tension of the wire undergoes successive variations with short period, resonance oscillation will take place to eventually lead to the breakage of the wire.